Wireless communications networks, and particularly cellular communications networks, have experienced tremendous growth in recent years. In a cellular communications network a geographic service area is divided into a number of cells. Each cell has a cell site (also called a base station) connected to a telephone network. The cell site establishes a wireless link with communications devices (hereinafter "user devices" or "devices") operated by network users within the cell who wish to send and receive information (e.g. text, audio, speech, video, etc.) via the public telephone network.
Current, first-generation cellular networks are based on analog FM technology in which a radio channel serves as the wireless link between a base station and a communications device. In these networks, interference between communications from different devices operating on the same channel is kept to acceptable levels by permitting each cell in the network to use only a subset of the radio channels available to the cellular network with the subset of radio channels for adjacent cells having no radio channels in common. System capacity is maintained through reuse of radio channels in non-adjacent cells. System quality is maintained by reusing frequencies only in cells that are far enough apart so that any interference is below acceptable levels. First generation cellular networks, however, are facing an increased demand for higher capacity to support a growing number of users and an increased demand for higher quality links to support data communications requiring low error rates.
With the objective of satisfying the demands for increased capacity and for higher quality links, second-generation cellular schemes, based on digital radio technology and advanced networking principles, are now being developed. These second-generation schemes will utilize spread spectrum multiple access techniques. Spread spectrum techniques are useful for facilitating communications when a large number of user devices wish to access a cellular network and when the transmissions in the network will be subject to interference. See, K. S. Gilhousen, et al., "On the Capacity of a Cellular CDMA System," IEEE Trans. Veh. Tech., Vol. 40, No. 2, pp. 303-312, May 1991; A. M. Viterbi and A. J. Viterbi, "Erlang Capacity of a Power Controlled CDMA System," J. Sel. Areas Comm., Vol. 11, No. 6, pp. 892-900, August 1993. By "spread spectrum" it is meant that each user generates a wideband signal (e.g. by code division multiple access or by very fast frequency hopping) which is treated as noise or interference by other users in the network. The design of spread spectrum cellular networks is challenging because the network must be reliable, with cellular networks typically assuring each user of a quality of service level, e.g. a guaranteed minimum bandwidth (in bits per second) and a guaranteed maximum bit error rate. The quality of service is typically related to the carrier-to-interference ratio (CIR), i.e. the ratio of the power level of a desired signal received at a given location to the power level of all other received signals at the given location.
Maintaining sufficient transmission quality on the wireless links is a critical factor in maintaining cellular system capacity and quality of service requirements. If for example, a signal transmitted by a communications device arrives at a base station at a power level that is too low, the bit error rate may be too high to allow high quality communications. If however, a signal transmitted by a communications device arrives at the base station at a power level that is too high, this high power level interferes with and degrades signals transmitted by other users. System capacity, therefore, can be increased if the transmitted power of each device is controlled such that the transmitted signal arrives at the base station at the minimum power level which satisfies the quality of service requirements.
The control of power levels of signals transmitted from devices to base stations may be either centralized or distributed. In centralized power control techniques, a single controller determines the power level for each device in the cell, and communicates that level to each device. Centralized control is advantageous in that a desired CIR level can be achieved immediately since the centralized controller has information about devices in contact with the base station (e.g. about which devices will terminate or initiate communications in a time interval). Distributed control, in contrast, uses an iterative approach in which power levels are adjusted based on feedback from the devices. Centralized control, however, involves the added infrastructure of a central control mechanism thereby resulting in added network vulnerability due to the single point of control.
Recent work has therefore emphasized distributed, or local, control. In a distributed power control network, the power level of each device is guided, using local measurements only, so that eventually all base stations meet any specified CIR requirements. Such power control methods typically adjust the power levels in communications devices based on a determination of the mean (which is a first order statistic) of the interference level at a base station. See, e.g., J. Zander, "Distributed Cochannel Interference Control in Cellular Radio Systems," IEEE Trans. Vehic. Tech., Vol. 41(3), pp. 305-311, August 1992; G. J. Foschini and Z. Miljanic, "A Simple Distributed Autonomous Power Control Algorithm and its Convergence," IEEE Trans. Vehic. Tech, Vol. 42(4), pp. 641-646, November 1993; and S. V. Hanly, "Capacity in a two cell spread spectrum network," 30th Annual Conference on Communication, Control and Computing, Allerton House, Monticello, Ill., pp. 426-435, 1992.
However, these methods of power control do not operate well in systems where there is randomness or variations in the received power level at the base station, such as in cellular networks where variations in the power received at a base station may be caused by a variety of factors, such as by multipath fading (caused by changes in the characteristics of the wireless link) or by communications devices transmitting information in bursts (as for example by a user transmitting information by speaking and by then remaining silent for a period of time before speaking again). Thus, there is a need for an improved method and apparatus to determine power levels for signals transmitted by communications devices in wireless networks.